I see a variety of German coaches made by BTTB, Tillig and the like, and I am wondering if there is one that has windows 6.6mm high x 6mm wide, with 2mm sections between the windows.

It would be better to get one that had windows inserts designed so the windows look flush with the sides of the coach. The railcar windows were flush with the car side, surrounded by a slightly raised rubberised seal.